The invention disclosed and claimed herein pertains to the field of neutron detection devices of the type which employ lithium-6, in a solid form, to respond to neutrons by radiating charged particles into an ionizable counting gas. More specifically, the invention pertains to detection devices of the above type wherein the lithium radiator is configured to provide one or more flat surfaces of lithium-6 foil in contact with ionizable gas, each surface being in spaced parallel relationship with an array of high voltage count wires which indirectly measure neutron activity by sensing pulses of gas ionization. Even more particularly, the invention pertains to detection devices of the above type wherein a plurality of flat lithium-6 foil surfaces may be stacked in parallel layers with one another, and with arrays of counting wires, in order to provide a neutron detector which is of very high sensitivity and which is yet capable of being contained in a package which is extremely portable, compact, and durable.
At present, most high-sensitivity neutron detectors of the radiator-ionizable gas type employ either .sup.10 BF.sub.3 or .sup.3 He, in a gaseous state, as the radiator medium for the detector, i.e., for the detector component which receives neutrons and which radiates ionizing particles in response thereto. .sup.3 He is always in a gaseous state at practical temperatures and pressures. .sup.10 BF.sub.3 must be employed in a gaseous state, since the principal ionizing particle which results from the reaction between a neutron and a boron nucleus of .sup.10 BF.sub.3 is an alpha particle, which is of extremely short range (e.g., 5.times.10.sup.-3 mm). If a reaction generating an alpha particle were to take place within a solid material, the dimensions of the material would have to be extremely small, to prevent the alpha particles from being trapped therewithin.
Because of the low density of .sup.10 BF.sub.3 and .sup.3 He at ordinary pressures, they must be contained in chambers of large volume in order to be used as the radiator component in a neutron detector. Consequently, such detectors tend to be comparatively large or bulky. While neutron detectors are available which have used a solid layer of .sup.10 B as an alpha particle radiator, the layer must be kept very thin, as aforementioned, (e.g., 10.sup.-2 mm) and it may still be necessary to supplement the .sup.10 B radiator with one of the above gaseous radiator components.
In the past, solid lithium-6 (.sup.6 Li) has been used as the neutron sensitive component in a radiator-ionizable gas neutron detector, wherein the .sup.6 L.sub.i is coated upon the curved inner surface of a cylinder. Note, for example, U.S. Pat. No. 2,721,944, issued Sept. 9, 1950, which discloses a neutron detector for use in geological exploration of oil fields. However, the Applicant has found that if, instead of such curved arrangement, a number of flat sheets of .sup.6 L.sub.i are employed in a neutron detector, it becomes possible to provide a substantial reduction in detector size and to increase the ruggedness thereof, and yet provide high detection sensitivity. The Applicant uses flat sheets of .sup.6 L.sub.i so that the sheets may be stacked in parallel layers within a thin, flat container. By enabling a number of sheets to be enclosed in the container, high sensitivity to neutron detection is provided since the probablility that a neutron entering the container will encounter a lithium-6 nucleus is optimized. By making the detector flat and thin, a neutron striking the detector from almost any angle will pass into at least one of the .sup.6 L.sub.i sheets. Also, by stacking sheets in parallel relationship, the space between sheets, which is filled with ionizable counting gas, may be made so small that gamma rays generated within the detector can be prevented from being registered as neutron counts. High-sensitivity neutron detectors employing the principles of the present invention have been fabricated which are small enough to be held in an operator's hand, and to be carried in a coat pocket.